【作者】 吴磊；

【导师】 肖安成；

【作者基本信息】 浙江大学 ， 构造地质学， 2011， 博士

【摘要】 长达1600km的阿尔金断裂是青藏高原的北界,它分隔了具有坚硬岩石圈的塔里木板块和相对较软的青藏高原,是高原上最重要、规模最大的走滑断裂,其新生代的演化一直是目前地质学界的研究热点之一,同时也是争论的焦点。本文利用位于阿尔金断裂中段南侧同时也是青藏高原内部面积最大的沉积盆地—柴达木盆地的沉积和构造响应来研究这一巨型的左旋走滑断裂在新生代以来的活动过程,并初步得出以下认识：(1)柴达木盆地西北缘新生代层序自下而上可明显分为三段：下段包括路乐河组和下干柴沟组(约36Ma之前),此时沉积环境稳定,以滨湖-浅湖相为主,沉积范围广阔,可能一直越过现今阿尔金山与塔里木盆地东南缘相接；中段包括上干柴沟组和下油砂山组,岩性为物源来自阿尔金山的河流相-冲积扇相粗碎屑沉积,剖面上构成向上变粗的磨拉石层序,说明此时阿尔金山进入了持续隆升的演化阶段,这一隆升过程的强度和范围在下油砂山组沉积末期(约15Ma)达到最大；上段由上油砂山组、狮子沟组和七个泉组组成,它们角度不整合于下伏地层之上,并向阿尔金山方向大规模超覆,反映阿尔金山的隆升幅度和范围在此时有所减小,其物源尽管也是来自阿尔金山,但却发生了很大的变化,暗示着阿尔金山在此之前发生过较大的构造变动。与此同时,柴达木盆地内部在新生代以来沉积环境一直处于稳定的接受沉积的状态,沉积物始终以湖相泥岩、粉砂岩为主,说明上述阿尔金山隆升的影响范围并没有到达盆地内部,仅局限在盆地西北缘紧邻阿尔金断裂的线性区域内。(2)柴达木盆地西北缘发育的近东西向断裂系统和盆地内部发育的北西向断裂系统并不是由于阿尔金断裂左旋走滑而形成的拖曳构造,而是不同期次、不同方向和不同性质构造的叠加。近东西向断裂系统分布在盆地西北缘紧邻阿尔金断裂并与之平行的狭长区域内,平均宽约30km,彼此呈雁列式排布,明显受控于左旋走滑；断面大多北倾,向下深切至基底,向上则一般仅切穿下油砂山组,少数能突破上油砂山组及以上地层,彼此构成叠瓦状向南逆冲,使盆地西北侧基底向南掀斜抬升；它们开始活动于下干柴沟组沉积末期(约36Ma),强烈活动于下油砂山组沉积末(约14.9Ma)。而北西向断裂系统则分布在盆地内部的广阔区域,控制着盆地内一系列背斜带的发育；断面倾向北东或南西,具有两层结构：深部为陡倾的基底断裂,形成宽缓背斜,浅层为反向的薄皮滑脱断层,在地表形成较紧闭背斜；开始活动于狮子沟组沉积初期(约8.2Ma),强烈活动于七个泉组沉积末期(约1.534-0.277 Ma)；形成于左旋应力背景下。可见,二者是在不同时间、不同区域、不同控制条件下形成的两套断裂系统,它们之间的转换标志着柴达木盆地乃至青藏高原北部的一次重要构造事件。(3)柴达木盆地与阿尔金断裂系统的耦合分析表明,阿尔金山早在约36Ma就开始隆升,而阿尔金断裂的大规模左旋走滑则发生在约15Ma以后。结合前人研究成果提出了阿尔金断裂新生代活动的两阶段模型：在36-15Ma,阿尔金断裂是一个局限在中下地壳的韧性剪切带,并在上地壳沿着青藏高原北缘(柴达木盆地和祁连山地区)与塔里木板块界线一带产生大范围的线性地表隆起和近东西向的基底断裂,形成阿尔金山的雏形；15Ma以后,阿尔金断裂突破地表开始大规模走滑,并对早期的阿尔金山进行改造,最终形成了现今的构造格局。现今阿尔金山主体的菱形形状是早期的古阿尔山祁连山段被后期的阿尔金断裂和北阿尔金断裂共同改造的结果。阿尔金断裂的左旋走滑把来自南侧的N-S向挤压应力转化为NE-SW向,导致了祁连山和东昆仑山显著的NW向线性隆升,这意味着高原北缘在新生代存在两个方向的变形构造：早期(36-15Ma)为近东西向,晚期(15Ma以后)为北西向,但前者由于被后者所强烈改造而仅在局部地区保留下来,柴达木盆地西北缘的近东西向断裂系统即为早期构造,而盆地内部的北西向断裂系统则为后期构造。更多还原

【Abstract】 The～1600km long Altyn Tagh Fault (ATF) bounds the Tarim plate with rigid lithosphere to the northwest and the Tibetan plateau with relatively weaker lithosphere to the southeast, and is the most important and the largest strike-slip fault inside the plateau. Its evolution during the Cenozoic not only has been all along the research focus for the geologists, but also provokes hot debates throughout the geoscience world. In this thesis, I use the sedimentary and structural records within the Qaidam Basin, the largest sedimentary basin inside the Tibetan plateau and located at the southern side of the ATF, as the proxies to study the Cenozoic tectonic process of this huge strike-slip fault. The conclusions are summarized as follows:(1) The Cenozic sequences of the northwestern Qaidam Basin are clearly divided into 3 units. The lower unit consists of the Lulehe and the Xiaganchaigou formations. During their depositing period (before ca.36Ma), the northwestern Qaidam Basin was a broad lake that may be across the present Altyn Shan and link to the southeastern Tarim Basin. The middle unit includes the Shangganchaigou and the Xiayoushashan formations, which are composed of fluvial-alluvial coarse-grained sediments with upward coarsening rhythm and provenance from the Altyn Shan, indicating the Altyn Shan continually uplifted since then and culminated at the end of the deposition of the Xiayoushashan formation (ca.15Ma). The upper unit consists of the Shangyoushashan, the Shizigou and the Qigequan formations, unconformably covers the underlying strata, and obviously overlaps towards the Altyn Shan, implying the range and altitude of the Altyn Shan both reduced at the time. Although the provenance of the upper unit was also the Altyn Shan, it differs greatly from that of the middle unit, suggesting a tectonic alteration occurring between the two units. However, inside the Qaidam Basin, the sedimentary environment had been all along lacustrine during the whole Cenozoic, and the sediments were fine-grained mudstone and siltstone, indicating the uplift of the Altyn Shan since ca.36Ma did not affect there and was restricted along the northwestern Qaidam Basin.(2) The EW-trending fault system in the northwestern Qaidam Basin and the NW-trending fault system inside the basin are completely different, and do not form curvilinear thrusting controlled by the left-slip on the ATF. The EW-trending faults distribute in a narrow belt (ca.30km wide) along the southern side of the ATF, and are in en-echelon arrangement, indicating that they were controlled by sinistral shear. These faults generally dip north, cut downward to the basement and mostly upward to the top of the Xiayoushashan formation. They thrusted imbricately to the south, tilting southward the basement of the northwestern Qaidam Basin. They started to be active at ca.36Ma, strongly activated at the end of the Xiayoushashan formation (ca. 14.9Ma), but became weak since then. The NW-trending faults distribute more expansively, and controlled the formation of anticlinal belts inside the Qaidam Basin. They dip NE or SW, and have two-layer structures:in the deep, they are steep-dipping basement-involved faults with wide anticlines in the hanging walls, while in the shallow layer, they are thin-skinned backthrust faults, forming close anticlines in the surface. They formed under sinistral transpressional settings, and started to develop at the beginning of the Shizgou formation (ca.8.2Ma), but strongly activated at the end of Qigequan formation (ca.1.534～0.277Ma). So they are two different fault systems forming at different periods, in different areas and under different controlling factors, and do not form curvilinear thrusting. The transition between them may mark an important tectonic event in the Qaidam Basin as well as the northern Tibetan plateau.(3) Coupling analysis between the Qaidam Basin and the Altyn Tagh Fault system proves that the Altyn Shan started to uplift as early as ca.36Ma, while large-scal left-slip motion on the ATF commenced since ca.15Ma. This suggests a two-stage model for the Cenozoic evolution of the ATF. During the early stage between ca.36～15Ma, the ATF was mearly a basal shear zone confined in the mid-lower crust, creating obviously surface uplift and EW-trending faults in the northern margin of the Tibetan plateau, which formed the rudiment of the Altyn Shan. During the late stage since ca.15Ma, the ATF eventually broke the whole crust, initiating large-scale left-slip movement and altering gradually the early Altyn Shan to the present framework. The diamond-shaped main body of the present Altyn Shan was part of the Qilian segment of the early Altyn Shan which was later cut and displaced by both the ATF and the North Altyn fault. Left-slip movement on the ATF transfers the N-S compressional stress from the south to NE-SW, leading to the prominent uplift of NW-trending Qilian Shan and Eastern Kunlun Shan. This suggests that there should be two types of structures in the northern Tibetan plateau during the Cenozoic: the early EW-trending structures forming during ca.36-15Ma, and the late NW-trending ones forming since 15Ma. The former was greatly altered by the latter and preserved only in some local regions, for instance in the Qiadam Basin, the EW-trending faults in its northwestern part belong to the early structures, while those NW-trending faults inside it are late structures.更多还原